The present invention relates to the general field of rotor-stator assemblies for a gas turbine engine. The invention relates more precisely to optimizing the microstructures of the abrasive coating and of the abradable coating that are deposited respectively on the tip of a rotor and on the inside surface of a stator surrounding the rotor.
A particular field in which the invention is applicable is that of compressors for aeroengine turbines (in particular for airplane engines or helicopter engines).
An aeroengine compressor comprises a compressor casing made up of one or more annular structures situated facing one or more sets of blades that move with relative rotation between the blades and the annular structures. Likewise, an aeroengine turbine comprises a turbine ring made up of an annular structure constituting a single piece or a plurality of touching ring sectors that surround a set of rotary blades driven by combustion gas.
In order to provide the best possible efficiency for an aeroengine, it is imperative to avoid, or at least minimize, leaks of gas between the tips of the blades and the facing surface of the compressor casing or of the turbine ring, since such leaks do not contribute to the operation of the engine. The search for no clearance or minimum clearance means that it is inevitable that contacts will occur between the tips of the blades and the facing material. Because of the hardness that is usual for this material, such contacts can be damaging to the tips of the blades.
In order to solve that problem, proposals have been made to optimize the abradability of the material situated facing the blades, i.e. its capacity to be worn away physically by the tips of the blades without giving rise to significant wear to the tips. One known technique for this purpose consists in providing the inside surface of the compressor casing or of the turbine ring with a layer forming an abradable coating made of porous material, while also providing the tips of the blades with a layer of dense material forming an abrasive coating.
For the abrasive coating deposited at the tips of the blades, it is thus known to have recourse to a material containing particles of cubic boron nitride (cBN) or of other hard particles of oxide, carbide, or nitride type. Such coatings present satisfactory abrasive behavior when they are associated with an abradable coating that is made of metal or of ceramic. Nevertheless, they present poor durability when they are used at high temperature (these coatings are subject to oxidation and to diffusion into the matrix of the coatings), and they also present thermomechanical behavior that is limited and not compatible with the looked-for lifetimes. Furthermore, it is relatively expensive to deposit such coatings.
For the porous abradable coating deposited on the inside surface of the stator, it is known to use a metal alloy of the MCrAlY type or to use a ceramic based on zirconia. A metal alloy presents the advantage of being easy to make at low cost. In contrast, its abradability properties are relatively poor and its use is temperature limited. Conversely, a zirconia-based ceramic can be used both to perform an abradable coating function and a thermal barrier function. The nature of the oxide-based coating enables it to be used at high temperatures (higher than 1100° C.), since it conserves its physical properties. This coating is also known for being easy to make, inexpensive (deposition by thermal spraying), and for a certain level of porosity, it presents good abradability properties. Nevertheless, such a porous coating presents surface state problems after machining because of the difficulty of shaping and controlling the sizes of the pores and of the grains in the layer. Unfortunately, such a surface state in the stream of air is harmful to the performance of the engine.
In general, although the abradable/abrasive coating pairs known in the prior art, all demonstrate a particular good property among the main looked-for properties (namely: behavior of the system on contacting, ability to withstand erosion, ability to withstand thermal cycling, and good surface state), none of them makes it possible on its own to optimize all of the looked-for properties.
There therefore exists a need to be able to have a pair of coatings available for the abradable material and for the abrasive material that are deposited respectively on the inside surface of the stator and on the tip of the rotor, that is capable of minimizing all of the above-mentioned drawbacks.